As is known in the art, surface acoustic wave (SAW)devices may be utilized to measure parameters which are proportional to strain induced in the devices by means of the coupling of the mechanical strain to the electric, surface wave properties of the devices. Specifically, the propagation velocity of a surface acoustic wave, and the length of the propagation path of a SAW delay line, are both functions of strain in the surface; thus, the operating frequency of a SAW delay line depends on strain in the surface thereof.
For instance, many piezoelectric materials such as quartz, lithium niobate, lithium tantalate, etc., exhibit the requisite acousto-electric coupling to provide a measurable variation in surface acoustic wave propagation velocity in response to variations in the subsurface strain thereof; additionally, composite treated substrates, such as silicon, having a suitable thin film coating of piezoelectric material, such as zinc oxide, exhibit the requisite piezoelectric characteristic for use in SAW transducers.
Any sort of strain-related phenomena such as temperature, stress, acceleration and other similar mechanical parameters, can be measured by suitably arranged SAW transducers employing a deformable piezoelectric material. Although the invention is not so limited, for simplicity in the dicussion herein, the following description is limited to pressure transducers in which the pressure difference across a piezoelectric diaphragm is measured through the variation in frequency of an oscillator including a SAW delay line on the surface of the diaphragm.
A variety of surface acoustic wave transducers are disclosed in U.S. Pat. No. 3,978,731. Therein, the strain responsive member, the mechanical effects of which are to be measured by the SAW delay lines, may comprise a diaphragm formed by removal of suitable material from a bulky substrate. The transducers disclosed in the aforementioned patent provide SAW delay lines on surfaces of that portion of the bulky material beyond the diaphragm, rather than on a surface of the diaphragm itself. It is believed that the summation of strain effects on wave propagation velocity across the entire diaphragm tend to sum vectorily to a small value, which value may provide so low a sensitivity as to render such devices impractical. It is also believed that the sensitivity of such devices to temperature variations may far exceed the sensitivity thereof to strain variations, and therefore mask the phenomena, such as pressure on the diaphragm, which is to be measured. As an improvement of such devices, U.S. Pat. No. 3,863,497 teaches mounting of the surface acoustic wave transducers on a surface of the diaphragm itself, rather than over the bulky substrate material. However, a SAW pressure transducer of the type described may have a frequency dependence on temperature of on the order of 20 or 30 parts per million per degree centigrade, whereas the strain sensitivity of frequency may be only about 6 or 8 parts per million per psi. Differential SAW transducers are known, such as in U.S. Pat. No. 3,848,144, and U.S. Pat. No. 3,863,497 (referred to hereinbefore), in which SAW delay lines are mounted on opposite sides of a piezoelectric wafer; but these do not lend themselves to practical, stable devices, particularly for sensing pressure and the like.